A variety of processes, involve the dissolution of gases in liquids. For example, in aerobic wastewater treatment systems, oxygen is required by the bacteria that enable the removal of the organic waste in the water. The required amount of oxygen can be effectively dissolved into process streams using mechanically agitated mixer systems. Similarly, in many drinking water treatment systems, chemicals such as flocculants and coagulants are added to facilitate the sedimentation and removal of contaminating solids in the liquid. Carbon dioxide is added for maintaining optimal pH conditions in some flocculation and coagulation processes by means of mechanically agitated gas dissolution systems.
Although surface mixing and gas dissolution systems are widely utilized, submerged mixing systems have certain advantages. One advantage is that such submerged mixing systems can be oriented, vertically, horizontally or at an angle for purposes of directional mixing or more optimal gas dissolution. Depth has a significant impact on effective system pressures; and the solubility of gases is known to be affected significantly by the pressure. Hence, a submerged mixing system when operated in water at a depth of ten meters can effectively enable the dissolution of about twice the volume of gas than would be possible if the mixing occurred at the surface.
A submerged mixing apparatus is disclosed in U.S. Pat. No. 6,273,402. The apparatus is adapted to be submerged in a tank that can contain waste water and sludge. In this device, a helical impeller connected to a motor rotates within a submerged draft tube. The draft tube and motor are connected to a ballast chamber that can be filled with ballast to cause the apparatus to sink within the liquid. The ballast chamber has a head space through which oxygen is injected and an undersurface that is provided with a slight slope towards inlet openings to the draft tube. The rotating impeller draws the liquid into the inlet opening for mixing with incoming oxygen. The resulting gas-liquid mixture is discharged from the other end of the draft tube in an expanding jet-like flow. Any oxygen bubbles that are not dissolved rise within the liquid toward the surface. A portion of these oxygen bubbles will be trapped by the undersurface of the ballast chamber and be entrained in liquid that is being drawn into the inlet openings. Another portion of the undissolved gas bubbles will escape from the surface of the liquid.
There are a number of drawbacks to the mixing and gas dissolution system described in U.S. Pat. No. 6,273,402. One major drawback is that the impeller itself is providing suction to draw the gas into the liquid. As the amount of the gas to be drawn increases, there will be less suction provided by the impeller and consequently, the amount of liquid that is able to be drawn by such a device will be limited, until eventually a condition known as flooding occurs, when no further liquid can be drawn in by the impeller.
Additionally, there is no control over bubble size of the gas bubbles that are produced by the action of the impeller. Although there are many factors that will have an influence on the dissolution rate of gas within a liquid within such a mixing device, for a given device subjected to particular operational conditions, the size of the bubbles will determine the interfacial surface area available for gas-liquid contacting and therefore, the amount of gas that is able to be dissolved in the liquid. Another major concern is the fact that the device shown in this patent can only be operated in a vertical orientation in that it depends on the underside of the ballast chamber in collecting gas that is not dissolved in the liquid for recirculation back to the draft tube, and relies on the horizontal orientation of the ballast chamber for maintaining stability at any given depth. In some water treatment systems it could be important not to direct the flow directly from the draft tube into the bottom of the treatment basin. If the basin is shallow and has an earthen bottom or polymeric lining, then the jet of liquid can damage the bottom surface or layer. However, even where the basin is made of a solid material such as concrete, if the liquid leaving the draft tube strikes the bottom at high velocity, before the bubbles disengage from the high velocity liquid, the bubbles will spread out a sufficient distance that will prevent the rising bubbles from being collected on the underside of the ballast chamber. This is exacerbated with large bubble sizes that will attain a sufficient terminal ascent velocity to escape from the surface of the liquid and avoid re-entrainment with the liquid being drawn into the draft tube.
There have been devices that do not have to operate in a vertical orientation. In one of such devices, the gas is injected through a ring-like manifold into the interior of the draft tube and below the impeller. This type of device has a series of openings that are each ⅛ of an inch. This produces large bubbles that will attain a terminal ascent velocity that is sufficiently high that a non-insignificant portion of the injected gas will escape from the surface of the liquid. Furthermore, the impeller used is not a helical impeller, but rather, a bladed impeller. There are limitations on the amount of gas that can be dissolved in such an apparatus because, as can be appreciated, as the amount of gas increases, there will not be enough liquid to be drawn and accelerated by the impeller.
As will be discussed, the present invention provides a method and apparatus in which the gas is injected directly into the draft tube with a controlled gas bubble size that will both enhance the degree to which the gas can be dissolved in the liquid and also the degree to which the undissolved gas will be entrained in the liquid flow being drawn into the draft tube by the impeller.